With reference to FIGS. 1 and 2, a conventionally general hydraulic press brake 201 has side plates 203L and 203R provided in a standing manner left and right, has an upper table 205U on upper front end surfaces of the side plates 203L and 203R so as to freely move upward and downward, and is provided with a lower table 205L on lower front surfaces of the side plates 203L and 203R in a fixed manner.
Further, hydraulic cylinders 207L and 207R for moving the upper table 205U upward and downward are provided in the upper front surfaces of the side plates 203L and 203R, and a hydraulic equipment 209 such as a pump, an oil tank, a control valve and the like for controlling each of the hydraulic cylinders 207L and 207R is provided in a rear portion of a center portion of the press brake 201 in a lump and is connected to each of the hydraulic cylinders 207L and 207R by a piping 211. In this case, a punch P is provided in a lower end portion of the upper table 205U so as to be freely replaced, and a die D is provided in an upper end portion of the lower table 205L so as to be freely replaced.
Accordingly, a bending work is applied to a work positioned between the punch P and the die D on the basis of a cooperation between the punch P and the die D by driving the respective hydraulic cylinders 207L and 207R according to a control of the hydraulic equipment 209 so as to move the upper table 205U upward and downward.
However, in the prior art mentioned above, since the respective cylinders 207L and 207R and the hydraulic equipment 209 are connected by the piping 211, a piping operation is required in an upper portion of the press brake 201. That is, since it is necessary to assemble the hydraulic cylinders 207L and 207R with the hydraulic equipment 209 according to an independent setup, and pipe them after mounting to a predetermined position, there is a problem that not only an operation man hour is increased, but also a risk is involved.
Further, since a long piping 211 is provided, there is a problem that the piping 211 is in danger of oil leak.
Further, since a distance between the control valve provided in the hydraulic equipment 209 and the hydraulic cylinders 207L and 207R is long, a response is bad, a pressure loss is great and a heat generation is accompanied, so that there is a problem that a controllability is bad.
Further, a working fluid in the oil tank is sucked into the hydraulic cylinders 207L and 207R by utilizing its own weight of the upper table 205U at a time of rapidly dropping, however, a temperature of the working fluid is low at a time of starting the process and a viscosity thereof is high, whereby a sufficient speed can not be obtained due to generation of suction lack or a cavitation is generated, so that there is a problem of a risk that a performance decrement is involved.
Further, a temperature of a frame is increased due to the heat generation, a heat deformation is generated in the frame, and a reduction of bending work accuracy is caused.
On the contrary, as shown in FIG. 3, for example, in a hydraulic cylinder 303 for moving an upper table 301U upward and downward corresponding to a ram in a conventional press brake, an upper cylinder chamber 305U of the hydraulic cylinder 303 is connected to a switch valve 311 by a piping 307, and a lower cylinder chamber 305L is connected to the switch valve 311 by a piping 309.
This switch valve 311 is connected to an oil tank 315 by a piping 313 and connected to the oil tank 315 via a hydraulic pump 319 by a piping 317. In this case, the hydraulic pump 319 is driven, for example, by an AC servo motor 321.
According to the structure mentioned above, in the case of moving a piston 323 upward, the hydraulic pump 319 is rotated by the AC servo motor 321, and the switch valve 311 is moved from a state shown in FIG. 3 to a left direction. Accordingly, a working fluid is discharged from the oil tank 315 by the hydraulic pump 319 so as to be supplied to the lower cylinder chamber 305L, and the working fluid in the upper cylinder chamber 305U is returned to the oil tank 315 through the switch valve 311.
On the contrary, in the case of moving the piston 323 downward, the hydraulic pump 319 is rotated by the AC servo motor 321, and the switch valve 311 is moved from the state shown in FIG. 3 to a right direction. Accordingly, since a flow of the working fluid is reversed, the working fluid discharged from the oil tank 315 is supplied to the upper cylinder chamber 305U, and the working fluid in the lower cylinder chamber 305L is returned to the oil tank 315 through the switch valve 311.
However, in the prior art mentioned above, since the working fluid in the upper cylinder chamber 305U and the working fluid in the lower cylinder chamber 305L are discharged to the oil tank 315 through the switch valve 311, a strong fluid force is applied to the switch valve 311 at a time of being discharged at a high pressure. Accordingly, there is a problem that an actuator having a great capacity is required for moving the switch valve 311, whereby a cost increase is caused and the switch valve 311 is enlarged.
Further, there is a case that the switch valve 311 vibrates due to a strong external force at a time of discharging the working fluid at a high pressure, and there is a risk that a vibration is generated in the upper table 301U corresponding to the ram. Further, as shown in FIG. 4, since a “opening degree−flow rate” property of the switch valve 311 is different between an unload time (a low pressure time) and a load time (a high pressure time), there is a risk that a motion gain of the ram is changed so as to generate the vibration. Accordingly, since the structure is made such as to make a ram speed at a time of depressure slow so as to reduce generation of the vibration, there is a problem that a productivity is deteriorated.
This invention is made by paying attention to the problems in the prior art mentioned above.
Accordingly, an object of this invention is to provide a press brake which can prevent an increase of oil temperature and can make an apparatus compact by widely reducing a capacity of an oil tank.
Another object of this invention is to provide a press brake which can reduce a shock at a time of depressure so as to prevent a vibration, and can increase a ram speed so as to improve a productivity.
Still another object of this invention is to provide a ram moving method in a press brake which can reduce a shock at a time of depressure so as to prevent a vibration, and can increase a ram speed so as to improve a productivity.